Chemical Retention Processes in Ponds

摘要

Small ponds are a ubiquitous component of the landscape, and have earned a reputation as effective water and chemical retention devices for surface water quantity/quality control. However, the chemical retention coefficient, R_i = (C_I - C_O)/C_I varies widely with time and among ponds such that authors have classified "keepers" (R_i > 0) and "leakers" (R_i < 0) with respect to total nitrogen (TN). These complexities suggest that time- and pond-specific compilations of R_i may have limited application. Here, we demonstrate the utility of the removal rate constant (λ_i in, e.g., day~(-1)) that separates the process responsible for chemical removal from the water residence time that determines the time over which such removal processes can act. Literature-reported monthly retention coefficients for two ponds have been recalculated as monthly removal rate constants, λ_i. The monthly removal rate constant is averaged and combined with the month-specific water removal constant (λ_w, inverse of water residence time) to significantly improve the pond-specific predictive capability. Second, internal loading of TN and total phosphorous (TP) ("leaker" periods) are obviously identified by R_i < 0 but are also identifiable when λ_i < λ_w. This identification of TP internal loading raises the likelihood of thermodynamic reduction of Mn and Fe that could release significant adsorbed metals. Comparison of removal rate constants to the removal rate constant for water and total suspended solids (TSS) allows a quick assessment methodology for the role of internal loading and particle settling in the control of specific chemical species.